How to add panel to 3D view Tool shelf in Texture Paint Mode
I use this

bl_&lt;prop&gt;      | allowed
---------------+-------------------------------------------------------------
bl_space_type  | 'CLIP_EDITOR', 'CONSOLE', 'DOPESHEET_EDITOR', 'EMPTY',
               | 'FILE_BROWSER', 'GRAPH_EDITOR', 'IMAGE_EDITOR', 'INFO',
               | 'LOGIC_EDITOR', 'NLA_EDITOR', 'NODE_EDITOR', 'OUTLINER',
               | 'PROPERTIES', 'SEQUENCE_EDITOR', 'TEXT_EDITOR', 'TIMELINE',
               | 'USER_PREFERENCES', 'VIEW_3D'
---------------+-------------------------------------------------------------
bl_region_type | 'CHANNELS', 'HEADER', 'PREVIEW', 'TEMPORARY', 'TOOLS',
               | 'TOOL_PROPS', 'UI', 'WINDOW'
---------------+-------------------------------------------------------------
bl_context     | 'armature_edit', 'curve_edit', 'imagepaint', 'lattice_edit',
               | 'mball_edit', 'mesh_edit', 'objectmode', 'particlemode',
               | 'posemode', 'sculpt_mode', 'surface_edit', 'text_edit',
               | 'vertexpaint', 'weightpaint'


So for you I think it's:

bl_space_type = 'VIEW_3D'
bl_region_type = 'TOOLS'
bl_context = "imagepaint"